The present invention relates to an arrangement for cooling gases used for a supercharged combustion engine.
The amount of air which can be supplied to a supercharged combustion engine depends on the pressure of the air but also on the temperature of the air. Supplying the largest possible amount of air to the combustion engine entails effective cooling of the air before it is led to the combustion engine. The air is usually cooled in a charge air cooler arranged at a front portion of a vehicle. At that location the charge air cooler has a cooling air flow at the temperature of the surroundings flowing through it, which makes it possible for the compressed air to be cooled to a temperature close to the temperature of the surroundings. In cold weather conditions, the compressed air may be cooled to a temperature below the dewpoint temperature of the air, resulting in precipitation of water vapour in liquid form in the charge air cooler. When the temperature of the surrounding air is lower than 0° C., there is also risk of the precipitated water freezing to ice within the charge air cooler. Such ice formation will cause a greater or lesser amount of obstruction of the air flow ducts within the charge air cooler, resulting in a reduced flow of air to the combustion engine and consequent operational malfunctions or stoppages.
The technique known as EGR (exhaust gas recirculation) is a known way of recirculating part of the exhaust gases from a combustion process in a combustion engine. The recirculating exhaust gases are mixed with the inlet air to the combustion engine before the mixture is led to the cylinders of the combustion engine. Adding exhaust gases to the air causes a lower combustion temperature, resulting inter alia in a reduced content of nitrogen oxides NOx in the exhaust gases. This technique is used both for Otto engines and for diesel engines. Supplying a large amount of exhaust gases to the combustion engine entails effective cooling of the exhaust gases before they are led to the combustion engine. The exhaust gases may be subjected to a first step of cooling in an EGR cooler which is cooled by coolant from the combustion engine's cooling system, and a second step of cooling in an air-cooled EGR cooler. The exhaust gases can thus also be cooled to a temperature close to the temperature of the surroundings. Exhaust gases contain water vapour which condenses within the EGR cooler when the exhaust gases undergo the second step of cooling to a temperature which is lower than the dewpoint of the water vapour. When the temperature of the surroundings is below 0° C., there is also risk of the condensate formed freezing to ice within the second EGR cooler. Such ice formation will cause a greater or lesser amount of obstruction of the exhaust gas flow ducts within the EGR cooler. When the recirculation of exhaust gases ceases or is considerably reduced, the result is an increased content of nitrogen oxides in the exhaust gases.